Dave Roche, the president of the Connecticut Building Trades, said construction unions played a role in helping Democrats regain outright control of the state Senate last year, only to grow increasingly angry as Senate Democrats refused to embrace any financing plan that included automobile tolls. Labor leaders say a failure by Democrats to pass an infrastructure plan would jeopardize what often is a perfunctory task: re-election endorsements of most Democratic lawmakers by the Connecticut AFL-CIO, the statewide federation of unions. Endorsements require a two-thirds vote. “Too often people clump all of organized labor together as this monolithic thing, and it’s not,” said Rep. Sean Scanlon, D-Guilford. “They are a big tent with different interests, and it’s important for us to remember that.”