Sen. Hillary Clinton has a slim lead over both former New York Mayor Rudy Giuliani and Sen. John McCain in a new national Gallup poll of presidential candidates.

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Clinton has larger leads over former Sen. Fred Thompson and former Gov. Mitt Romney, Gallup said.

Sen. Barack Obama also enjoys a wide margin over Thompson and Romney, but more or less ties with Giuliani and McCain.

The poll of 897 registered voters nationwide was conducted Nov. 11-14.

Clinton's standing against Giuliani, McCain, and Romney has remained about the same since similar polls during the summer. She now pulls greater numbers against Thompson.

Obama has not improved his standing against any of the leading Republican candidates compared to poll results from the summer, Gallup said.

Leaving aside undecided voters, the Clinton match-ups break down this way:

• Clinton 49 percent, Giuliani 44 percent
• Clinton 50 percent, McCain 44 percent
• Clinton 53 percent, Thompson 40 percent
• Clinton 54 percent, Romney 38 percent

The Obama match-ups break down this way:

• Obama 45 percent, Giuliani 45 percent
• Obama 47 percent, McCain 44 percent
• Obama 51 percent, Thompson 38 percent
• Obama 52 percent, Romney 35 percent

Clinton and Obama both began long days of campaigning in Iowa on Sunday by attending church services.

Before the service at Grace United Methodist Church, Clinton stood in the lobby and greeted dozens of people, shaking hands and posing for photographs. She was greeted by warm clapping during the children's service.

In her sermon, the Rev. Jill Flyr asked the congregation to "let the candidates know that they need to be strong advocates for children."

Obama spoke at Union Baptist Church in Des Moines.