Obama hopes to pass key marker

(CNN) -- U.S. presidential hopeful Barack Obama is expected to declare he has passed a key milestone in the fierce race with Hillary Clinton to be the Democratic Party candidate.

Voting is taking place Tuesday in Kentucky, where Clinton is ahead in opinion polls, and Oregon where Obama is leading and is expected to notch up a majority of pledged delegates.

In U.S. Democratic races, votes won are converted proportionally into pledged delegates and are vital to winning the nomination, but in this race it is highly unlikely either candidate will get enough pledged delegates for automatic victory.

"We still have some contests left, but if Kentucky and Oregon go as we hope, then we think we will have a majority of pledged delegates, and that's a pretty significant mark," Obama said

In turn that means, he will be able to claim for the first time a majority of pledged delegates, meaning Clinton cannot overtake him on that scorecard.

He has also won more states and he is ahead in the race to secure superdelegates -- party officials who are not tied to the primary voting and are free to back whichever candidate they want.

Even if Obama has a poor showing Tuesday, he is expected to easily get the 15 delegates needed to have won a majority of pledged delegates.

Kentucky has 51 delegates at stake and Oregon 52 and they are distributed largely proportional to the percentage of the vote received.

Obama said: "I won't be the nominee until we have enough -- a combination of both pledged delegates and superdelegates -- to hit the mark. But what it does mean is that voters have given us the majority of delegates that they can assign. And obviously that is what this primary and caucus process is about."

Clinton was campaigning Tuesday in Kentucky while Obama was in Iowa, one of the key battleground states come the November presidential election and which President Bush won by just 10,000 votes in 2004.

Clinton is arguing she has the lead in the popular vote but only once you have taken out caucus results, ostensibly because of the low turnout at caucuses but which were mostly won by Obama, and then add in Michigan and Florida -- whose results the party decided would not be counted because the states broke party rules. In Michigan, Clinton was the only big name candidate on the ballot.

Caucusing is a different way of choosing a candidate where party supporters debate before deciding who they will support as a group rather than the traditional ballot card method used in primary states. Read our election Jargonbuster

She also says she has won better states -- from a Democratic point of view -- because some of the states Obama won are likely to vote Republican in November's general election whereas she has won the Democratic core states.

The CNN poll of polls, which averages the latest public opinion surveys in the state, suggests Clinton leads Obama by 30 points. Polls in Kentucky close at 7 p.m. ET (2300 GMT)

Obama is the favorite in Oregon, where a CNN poll of polls indicates he has a 10-point lead over Clinton. Oregon's primary is a mail-in only contest, which means voters there need to mail in or hand in their ballots in person by 8 p.m. PT (0300 GMT Wednesday).

Obama spent the weekend campaigning in Oregon while Clinton spent four straight days in Kentucky. Both candidates have been speaking on the economy, health care, and the war in Iraq.

They also criticized John McCain, the presumed Republican presidential nominee, by linking him to President Bush. But Clinton and Obama have avoided mentioning each other.

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