The obvious names were the preliminary ones listed Tuesday for Oregon’s presidential primary ballot May 15.

Secretary of State Kate Brown listed the four Republicans still in the field — Mitt Romney, Newt Gingrich, Rick Santorum and Ron Paul. President Obama is the only Democrat.

Oregon law leaves it to the secretary of state to determine who is “generally advocated or recognized” in the national news media as presidential candidates. A candidate can qualify for the presidential ballot, however, by obtaining 1,000 voter signatures from each of Oregon’s five congressional districts and filing them with the secretary of state.

“I want Oregonians to have ample time to educate themselves on the candidates,” said Brown, who is a Democrat.

The list is made final after the primary filing deadline March 6.

Under Oregon law that dates back more than a century, only registered Democrats can vote for Democratic presidential nominees, and registered Republicans for Republican presidential nominees. April 24 is the deadline for voter registration for the primary.

pwong@StatesmanJournal.com or (503) 399-6745 or twitter.com/capitolwong