Hillary Clinton and her supporters remained defiant Tuesday as Barack Obama appeared ready to capture an insurmountable majority of pledged delegates once the results of the Kentucky and Oregon primaries are known.

Obama, who has increasingly been focusing his attention on presumptive GOP nominee John McCain, arranged an evening appearance in Iowa, site of his critical Jan. 3 caucus triumph. He will likely announce he's won most of the delegates at stake in all 56 contests on the campaign calendar.

But the Clinton campaign saw the Democratic race differently.

Campaigning with his wife in Kentucky, former President Bill Clinton dismissed the claim that Obama would clinch the majority of pledged delegates.

"There won't be tonight, unless you decapitate Michigan and Florida, which violates our values and is dumb politics," the former president said.

Click here to see photos from the Kentucky and Oregon primaries.

Hillary Clinton hopes to finish with more votes than her rival in all the contests combined, including Florida and Michigan, which were stripped of their delegates for holding their primaries before Feb. 5.

"This is nowhere near over," she said on Monday.

Clinton will likely get prime-time coverage Tuesday night. Favored in all polls to win Kentucky, she has scheduled a rally in Louisville for 8 p.m. ET, one hour after the voting ends in that state.

Obama will not speak in Iowa until more than two hours later. First results in Oregon, where he is favored, are not expected until 11 p.m. ET.

In a full page ad in The New York Times, Clinton's female supporters demanded she stay in the race despite overwhelming odds.

"We want Hillary to stay in this race until every vote is cast, every vote is counted, and we know that our voices are heard," said the ad, paid for by the WomenCount political action committee.

But the nomination is rapidly moving out of Clinton's grasp.

A new Gallup national tracking poll suggests that voters are beginning to accept Obama as the inevitable Democratic nominee.

The poll, taken from May 16-18, showed him with a record 16-point lead over Clinton -- that's compared with a 4-point lead over Clinton in early May.

The poll also showed Obama was starting to eat into Clinton's base. Though she normally pulls women, Hispanics, seniors and other groups, the only major group still supporting her by 51 percent or more in the Gallup poll was women who are 50 years old and up. The poll was conducted of 1,261 Democratic and Democratic-leaning voters.

As of Tuesday morning, Obama was only 17 delegates short of reaching a majority of the 3,253 pledged delegates available in all state contests. With 103 delegates at stake in Tuesday's primaries, it was practically guaranteed Obama would achieve what he called a "milestone."

"You know I think that we have been very restrained in this whole process," Obama told FOX News Tuesday. "We are going to reach a milestone today, hopefully, that is very important.

"We haven't declared victory. We have got three more contests after tonight and we still are going to have to get more superdelegates in order for us to actually secure the nomination."

Before vote counting began, Obama had 1,917 overall delegates, little more than 100 shy of the 2,026 needed to become the nominee. The former first lady had 1,722.

Obama weeks ago predicted Clinton would win Kentucky. Clinton scored a wide victory in West Virginia last Tuesday and polls show her leading Kentucky by double digits. The Bluegrass State offers 51 pledged delegates, which will be divided proportionally.

The Clinton campaign similarly indicated that Obama would likely win Oregon, where Obama invested his time and drew a crowd estimated at 75,000 over the weekend

The Beaver State has 52 delegates. It also had the distinction of staging the only contest without a designated polling day.

Instead, under a vote-by-mail system, election officials tallied all ballots received by 11 p.m. on primary day.

The only primaries remaining are Puerto Rico, on June 1, followed two days later by South Dakota and Montana.

Obama has steadily picked up superdelegate endorsements since his double-digit win in North Carolina two weeks ago. He recently eclipsed Clinton's lead in superdelegates.

Fundraisers for the Obama and Clinton campaigns have held quiet discussions on working together in the fall campaign. Additionally, Obama's top strategist, David Axelrod, disclosed he had contacted Clinton's former campaign manager, Patti Solis Doyle, about joining forces for the general election.

Solis Doyle confirmed what she called informal conversations about how she might help the Illinois senator if, as expected, he secures the presidential nomination.

FOX News' Aaron Bruns and Bonney Kapp and The Associated Press contributed to this report.