DES MOINES — A crowded field of potential Republican presidential candidates scrapped for the hearts of the party’s conservative base here Saturday, implicitly rejecting the more moderate wing favored by big donors and trying to fire up the kind of grass-roots supporters who will play a critical role in the nominating process.

The daylong forum, billed as an informal kickoff to the 2016 campaign, was attended by about 1,200 people, many of whom ardently oppose the centrist views that tend to prevail in a general election.

The speakers, some of them experienced presidential campaigners, came to test and tweak their messages, to seek second chances and to introduce themselves to voters whose passion for conservative causes makes them more likely to attend a caucus and launch a candidate out of a field of contenders.

Two likely candidates who did not attend, Jeb Bush and Mitt Romney, were criticized by several who did — mostly in veiled swipes, rather than by name — over the Common Core educational standards and immigration reform, which Mr. Bush in particular supports.