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Roberts said Thursday he will vote against Sotomayor.

WASHINGTON (CNN) - Sen. Pat Roberts, R-Kansas, said Thursday he does not plan to vote to confirm Supreme Court nominee Sonia Sotomayor, becoming the first Republican to explicitly state his opposition to President Obama's pick for the high court.

"With all due respect to the nominee and nothing personal, I do not plan to vote for her," Roberts told talk radio host Christ Stigall on Kansas station KCMO.

Roberts also noted he was one of the 28 Senate Republicans in 1998 who voted against confirming Sotomayor to the Second Circuit Court of Appeals. Sotomayor was ultimately confirmed to that court by a 68-28 vote.

"I did not feel she was appropriate on the appeals court," Roberts said of his 1998 vote. "Since that time, she has made statements on the role of the appeals court I think is improper and incorrect."

"I think that we should be judging people not on race and gender, or background or ethnicity or a very compelling story," Roberts continued. "There are a lot of people who have that."

Roberts is skipping his party's convention.

(CNN) – Another Senate Republican is officially skipping the party's national convention next month, the latest sign vulnerable Republicans may be wary of linking themselves to the GOP brand this election cycle.

Pat Roberts, the two-term senator from Kansas who finds himself in a closer re-election bid than initially expected, won't be on hand to see Sen. John McCain accept the Republican presidential nomination in Minneapolis, his spokesman Molly Haase confirmed to CNN.

Roberts will instead be completing a 105-county tour of the state, Haase said. Polls show Roberts holds a double digit lead over Democratic challenger Jim Slattery, though the race has narrowed over the last several weeks.

Roberts joins a handful of vulnerable Senate Republicans who have flatly said they will not attend the convention, including Susan Collins of Maine, Gordon Smith of Oregon, Ted Stevens of Alaska, and Elizabeth Dole of North Carolina.

Two other vulnerable Republicans - New Hampshire Sen. John Sununu and Mississippi Sen. Roger Wicker - have yet to announce if they will attend.

Roberts' decision comes a week after Republican Congressional Committee Chair Tom Cole reportedly suggested some embattled Republicans should steer clear of the GOP Convention.

According to a report in The Hill, the Oklahoma congressman also urged Republican lawmakers to feel free to criticize the party -– including prospective future colleagues - citing dismal congressional approval ratings. “These ratings are worse than we had on the eve of losing the majority,” he said. “Don’t be afraid to say you are disappointed in fellow Republicans… don’t hesitate to be anti-Washington, D.C."