Americans for Prosperity Colorado announced plans Wednesday to rain down pressure on four Republican senators who have yet to state a position on a bipartisan transportation funding package unveiled last week by legislative leadership.

The conservative advocacy organization’s state director, Michael Fields, said the four GOP lawmakers – Senate President Pro Tem Jerry Sonnenberg, R-Sterling, and state Sens. Bob Gardner, R-Colorado Springs, Kevin Priola, R-Brighton, and Jack Tate, R-Centennial – can expect “tens of thousands” of phone calls urging opposition to House Bill 1242, which proposes asking voters to approve a nearly $700 million state sales tax hike to pay for road construction and other transit needs.

“Key state senators have yet not come out in opposition to the massive statewide sales tax increase, and their constituents deserve to know where they stand on House Bill 1242,” Fields said in a statement.

He told The Colorado Statesman that more lawmakers could be added to AFP’s target list in the near future.

The bill has faced a fierce backlash from Republican legislators, although two leading GOP senators are among its chief sponsors.

The legislation would refer a ballot question to voters in November asking for an additional 0.62 percent added to the state sales tax , bringing the rate to 3.52 percent, for 20 years. Proceeds would finance $3.5 billion in transportation bonds, as well as distribute funds to counties and municipalities for road work and pay for “multimodal” transportation projects around the state. If approved, the referendum would also return $75 million a year in vehicle registration fees to taxpayers.

Its initial sponsors are Senate President Kevin Grantham, R-Cañon City, and House Speaker Crisanta Duran, D-Denver, and the chairs of both chambers’ transportation committees, state Rep. Diane Mitsch Bush, D-Steamboat Springs, and state Sen. Randy Baumgardner, R-Hot Sulphur Springs.

“One of our primary purposes as an organization is to inform citizens regarding policies that impact their economic freedom and to hold elected officials accountable – we will do that regarding this proposed $700 million tax increase,” Fields said.

The bill has its first scheduled hearing in a week, before the House Transportation and Energy Committee on Wednesday, March 22.

Anti-tax crusader Grover Norquist’s Americans for Tax Reform organization came out swinging against the proposed legislation on Monday, urging Colordo lawmakers to oppose the bill.

“Some lawmakers contend this regressive tax hike is needed because transportation is a priority,” Norquist, president of Americans for Tax Reform, wrote in a letter to legislators. “Yet lawmakers calling for a tax hike to fund transportation are actually admitting that transportation is their lowest priority. Were that not the case, they would not have funded everything else in the budget first.”

In addition, the conservative Independence Institute has filed a proposed ballot measure called “Fix Our Damn Roads,” which would ask voters to approve $2.5 billion in transportation bonds to be financed with existing state revenues.

– ernest@coloradostatesman.com