Sen. Barbara Boxer used an earthmover as a backdrop Wednesday for what has become her weekly public push for a transportation bill, but she used a blunt instrument to deliver her message.

In a sharp turn of tone, Boxer and a half-dozen other Democratic senators — joined by construction and union leaders — described the Republicans who have objected to conference committee proposals as extremists who are holding the bill hostage for political gain, want to scuttle transportation funding and should hang their heads in shame.

“There is only one group standing in the way of the bill, only one group standing in the way of 3 million jobs,” Boxer said. “Those are the House Republicans. They are standing in the way of progress.”

The conciliatory tone that had characterized Boxer’s past public statements evaporated after construction and union leaders emerged disappointed Wednesday morning from a meeting with a key staff member of House Speaker John A. Boehner (R-Ohio).

Though the House is not in session this week, conference committee members have continued to swap proposals aimed at reaching agreement on the latest iteration of a $109 billion, two-year transportation bill passed by the Senate last year. Transportation funding is now operating under a temporary extension that expires June 30.

Boehner was not able to get the House to agree on its own bill, and he appears to have struggled in getting some of his members to support the conference committee proposals. He suggested last week that a six-month extension was under discussion.

“It’s not all Republicans,” Sen. Charles E. Schumer (D-N.Y.) said at Boxer’s news conference. “We had great Republican support in the Senate. Nor is it all Republicans in the House. But there are 100 people — militants, radicals, extremists — who actually believe the federal government should not be involved in highways.”

The earthmover and four cement mixers that flanked it were props Boxer employed to emphasize that the transportation bill would put millions of unemployed construction workers back on the job.

The Republicans have linked three controversial non-transportation issues to the bill: expansion of offshore and arctic oil exploration, approval of the Keystone pipeline and relaxation of restrictions on use of coal ash.

Boxer said resolution of those issues was in reach if agreement could be reached on the key transportation issues. She declined to reveal which issues had become sticking points in the talks.

“We are at a crossroads,” said a key staff member involved in the negotiations who asked not to be named because of the sensitivity of the talks. “It has to be pretty much done by next week because we have to get a conference bill approved by both houses.”

As construction and labor groups renewed their public push for passage at the Capitol Hill rally, they also launched a radio advertising campaign in the home states of four Republican House members who are on the conference committee.

Reps. Dave Camp (Mich.), James Lankford (Okla.), Steve Southerland II (Fla.) and Patrick J. Tiberi (Ohio) were targeted in ads that were identical in their content but personalized to add the number of bridges in need of repair in each respective state.

Each ad, sponsored by a group called the Transportation Construction Coalition, asked whether the named congressman would “be part of the problem” by opposing a compromise.

Rep. John L. Mica (R-Fl.), chairman of the House Transportation Committee, said in a statement: “I am disappointed in the fact that Senate negotiators have yet to move significantly on key House reform proposals. In addition, the Senate leadership appears unwilling to compromise at all on the Keystone XL pipeline."