WASHINGTON— Declaring himself a proud conservative before a crowd of cheering supporters waving American flags, Senator Sam Brownback, Republican of Kansas, on Saturday announced his candidacy for president.

Mr. Brownback, an evangelical Protestant turned Roman Catholic and a former Kansas agriculture secretary, said he would focus on reviving faith and families in America; combating abortion, poverty and wasteful government spending; opposing same-sex marriage; and revamping Social Security and the federal tax system.

“The last thing we need in America is to take God out of our public lives and institutions,” Mr. Brownback said during his speech in Topeka, Kan. “We need to embrace our nation’s motto, ‘In God we trust,’ and not be ashamed of it.

“To walk away from the Almighty is to embrace decline for a nation,” he said. “To embrace him leads to renewal, for individuals and for nations.”

Mr. Brownback, 50, was first elected to the House in 1994. Two years later, he was elected to the Senate, to complete the unexpired part of Senator Bob Dole’s term when Mr. Dole left to run for president.

Mr. Brownback acknowledged that he would be one of the lesser-known candidates in what is expected to be a crowded field of Republican presidential hopefuls, including Senator John McCain of Arizona and Rudolph W. Giuliani, the former mayor of New York.

He said his bid might be “a long shot,” but he compared himself to other successful presidential candidates who had come from behind, including Jimmy Carter and Bill Clinton. He said his ideas would separate him from the pack.

“I start out with less name ID, but I’ve been there before,” Mr. Brownback said at a news conference after his speech in Topeka, Kan. “My positions are at the heart of where the Republican Party is.”

With his opposition to abortion, embryonic stem-cell research and same-sex marriage, Mr. Brownback hopes to establish himself as the dominant conservative in the race. The American Conservative Union, which hails him as one of “the best of the best” in the Senate, gave him a 100 percent conservative rating in its most recent survey of Congress.

In recent weeks, Mr. Brownback has been emboldened by the struggles of Mitt Romney, the former Massachusetts governor, whose conservative credentials have come under attack because of questions about his evolving positions on abortion and gay rights. Mr. Brownback said Saturday that he planned to march in an anti-abortion rally in Washington on Monday.

But Mr. Brownback has also broken with Republicans on some issues. He supports putting most of the nation’s 12 million illegal immigrants on a path to citizenship, for instance. And this month, he joined many Congressional Democrats in announcing that he opposed President Bush’s plan to increase the number of troops in Iraq.

Paul M. Weyrich, chairman of the Free Congress Foundation, a conservative research group, praised Mr. Brownback for his commitment to conservative principles. But he questioned whether the senator had the charisma to prevail. “If he can prove that, then he will have a shot,” Mr. Weyrich said. “If not, then his conservatism will not be enough to carry him.”