DENVER -- Colorado caucus goers begin the process of choosing candidates for their party's nominations Tuesday night.

Thousands of people will gather in homes, schools or public buildings across the state to start choosing candidates for the November elections.

President George W. Bush and Sen. John Kerry have already secured the nomination for their parties so the focus is on local and state races.

Republicans and Democrats will participate in an estimated 3,500 caucuses, where they will nominate candidates, write a platform and organize the effort to get out the vote on Election Day. They'll also pick delegates to send to the upcoming statewide convention, which in turn will choose which Senate candidates get on the primary ballot in August and who will go to the national political conventions.

This year's caucuses gained importance even before Republican Sen. Ben Nighthorse Campbell said he wouldn't seek re-election.

Lawmakers voted to eliminate the state's presidential primary this year to save money. So Colorado's delegates to the presidential nominating conventions will be chosen by the caucus system for the first time since 1988.

To get their names on the August primary ballot, U.S. Senate candidates have to get the support of at least 30 percent of the delegates at the state conventions, which will be held June 5 for Republicans and May 21-23 for Democrats. Candidates who get at least 10 percent of the vote at a convention can still get on the ballot if they gather enough voter signatures. They could also bypass the caucus system entirely by also collecting petition signatures.

This year, Colorado Democrats will send 64 delegates and nine alternates to the national convention in Boston. Republicans will send 50 delegates and 47 alternates to New York.

Colorado lawmakers may move the 2008 caucus up to February, which would give Colorado voters more of a say in the presidential selection process.