House Republicans blocked the General Assembly on Tuesday from reaching the supermajority necessary to seek a referendum vote next year on a state constitutional amendment to create a legal ‘lockbox’ to protect future transportation funding.

The Senate voted unanimously for the resolution placing the question before voters, but the House vote for the measure was 100 to 40 — 14 votes short of the 114 needed to place the proposed amendment on the fastest track for approval allowed under the state Constitution.

House Minority Leader Themis Klarides, R-Derby, who participated in bipartisan talks preceding Tuesday’s special session on a deficit-mitigation plan and other fiscal issues, voted for the resolution, but 37 of the 64 members of her caucus did not. Three Democrats also voted no.

The vote was a setback to Gov. Dannel P. Malloy, who saw quick adoption of a constitutional lockbox as central to winning public and legislative support for what he says are major overdue investments in transportation infrastructure.

But several speakers, including a Democratic leader, Deputy House Speaker Bob Godfrey of Danbury, said the lockbox amendment was more concept than finely tuned constitutional amendment.

Under the resolution, lawmakers still would have several months to decide what resources would be “locked” away for transportation before voters ultimately vote on the Constitutional amendment.

With legislators still weighing Malloy’s call for a landmark, 30-year expansion of transportation investment, Senate leaders from both parties hailed the ‘lockbox’ concept.

“As we will be taking a stepped up commitment to transportation, I think this provides a system and a framework to do so with the utmost confidence” that resources earmarked for transportation will be spent on that purpose alone. said Senate President Pro Tem Martin M. Looney, D-New Haven.

The amendment “has been a long time coming and deeply appreciated,” Sen. Toni Boucher of Wilton, ranking GOP senator on the Transportation Committee, said.

The new amendment states that, if ratified by voters, all resources designated for transportation will remain for that purpose for as long as the state collects or holds them.

But that doesn’t mean the lockbox system is simple.

For example, Connecticut currently levies a 25-cents-per-gallon retail tax on gasoline, and the entire $511 million it raises currently is spent within the Special Transportation Fund.

But if the legislature were to add a few pennies to the gasoline tax in a future year — and raise more than $511 million — the extra funds would not automatically lie within the legal ‘lockbox’ and could be spent for non-transportation purposes.

Looney said he expected the legislature would include “traditional” sources of transportation funding, such as gasoline taxes, in the lockbox at their existing levels.

But as far as how the proceeds from potential future tax increases are spent, that “will continue to be a legislative matter from session to session,” Looney said.

Another wrinkle in the ‘lockbox’ system is that the earliest it could have taken effect, even if the House had endorsed it by a super majority, was Nov. 8 of 2016.

If a legislative resolution launching the amendment passes both the Senate and House with at least three-quarters of the votes cast, it then goes before voters for final ratification in the next statewide election – which is in November 2016.

If it passes in both chambers – but fails to secure the three-quarters’ margin in at least one – it could be reconsidered by the General Assembly in 2017.

If it then passes both chambers of the legislature for a second time – by any margin – it would go before voters on the November 2018 ballot.

The routes to adoption now are varied: a modified version, if endorsed by a three-fourths vote next year, still go on the ballot in 2016, but getting the same amendment before voters is not possible before 2018.

Besides the retail gasoline tax, the transportation fund also receives monies at this time from a wholesale fuel tax, the sales tax, certain motor vehicle and other fees, and from federal grants. But that doesn’t mean they all would be in the ‘lockbox’ if and when voters ratify the amendment.

According to the system, only revenues assigned by law to the transportation fund on the date of ratification are locked in place.

But the regular 2016 legislative session, which runs from early February through early May, will transpire before then. This would give legislators several months to amend which revenue streams are assigned to the Special Transportation Fund.

A special panel studying options to finance transportation improvements in the long-term isn’t due to report recommendations until January. Among the options the group has studied are gasoline tax increases and the restoration of tolls.

But with legislators facing a state election year, some lawmakers have questioned whether they will decide any long-term transportation funding questions until 2017.

R. Nelson “Oz” Griebel, president of the MetroHartford Alliance, wrote Monday to the governor and legislators, arguing that while a constitutional lockbox remains crucial, officials should wait to act on one until after the Transportation Finance Panel finishes its work in January.

Godfrey, one of three Democrats to vote against the resolution, told his colleagues during the debate that Griebel’s letter was persuasive. Off the House floor, he questioned the need for a vote in special session without a public hearing.

“I know it’s really boring to talk about process, but you need to have this exposed to the public,” he said.

Others acknowledged the measure’s flaws, but urged passage.

‘“We’ll take half a loaf when we can get it,” Senate Minority Leader Len Fasano, R-North Haven said, adding he remains concerned that the amendment “doesn’t go far enough to ensure” resources clearly earmarked for transportation are used for that purpose.

Klarides, the House GOP leader, indicated her vote was a close call.

The ‘lockbox’ “is a Republican concept from several years ago,” Klarides said after the vote. “I’ve always been happy with the concept and I would have preferred that the definition had been stronger.”

On the other hand, Klarides added, there also was some value in establishing some controls on transportation funding now. “I certainly understand both sides,” she said.

Reaching the three-fourths majority meant that the House GOP minority was crucial. If the 10 missing Democrats were present and voted yes, the resolution would have been four votes short of the necessary 114. If the three dissenting Democrats were persuaded to switch, the tally would be been 113, still one short.

The House Democrats voting no with Godfrey were Catherine F. Abercrombie of Meriden and Gregory Haddad of Mansfield.

Rep. Arthur O’Neill, R-Southbury, offered an amendment that he says would have tightened the lockbox definition and make clear that the state Supreme Court had original jurisdiction over any issues of enforceability.

“It will keep us honest,” O’Neill said.

Rep. John T. Shaban, R-Redding, said, “I think this gets us over the goal line with everybody on board.”

O’Neill’s amendment failed on a party-line vote.