DES MOINES (AP) - Leaders of several prominent conservative groups in Iowa warned potential presidential candidates on Friday that "our members will be watching" confirmation hearings for Supreme Court nominee John Roberts.

"The manner in which you conduct yourselves during this process will help steer the tenor of the Senate and potentially of the next presidential campaign," the activists said in a "Dear Senator" letter aimed at lawmakers pondering a run for president.

"We intend to follow this process closely," the letter said.

The groups endorsing the letter range from the Iowa Christian Coalition, the Iowa Family Policy Center, Iowans for Tax Relief and 11 Republican members of the Legislature. The activists have enormous influence over Republican politics and their ire could doom a presidential candidate among the GOP.

Iowa has a heavy influence on the presidential selection process because its precinct caucuses launch the presidential primary calendar. Potential candidates of both parties already have begun courting activists with an eye on the 2008 election.

"As leaders of organizations with politically active Iowa members, we applaud all of you for moving ahead with floor votes on qualified judicial nominees last spring," the letter said. "Now, our members will be watching the progress of Judge Roberts' confirmation."

The letter urged "a fair hearing and a floor vote in a timely manner."

"We want to make it clear to all erstwhile presidential candidates," said Des Moines lawyer and 2002 GOP gubernatorial candidate Doug Gross. "If they filibuster that nomination, they're going to be held accountable."

The letter was released at a Statehouse news conference that was the latest step in a carefully coordinated campaign to build momentum for the Roberts nomination. The letter was signed by 29 key conservative activists.

Des Moines lawyer and Polk County GOP Chairman Ted Sporer said the activists won't hesitate to use the state's leadoff role to bring pressure on those pondering a run for president.

"Iowa has a unique position in the nation's political landscape," said Sporer.

Chuck Hurley, a former lawmaker who runs the Family Policy Center, said social conservatives will be monitoring hearings to determine Roberts' position on abortion.

"Clearly, abortion is at the center of this process," said Hurley.