Sen. Daniel J. Evans (R-Wash.) said yesterday that he will switch sides on the controversial appeals court nomination of Daniel A. Manion, leaving Senate Democrats convinced that they are one vote short of defeating the nomination in a scheduled vote today.

Evans confirmed earlier reports that he would vote with the Republican leadership on a procedural move to kill a second vote on the nomination. "I voted against him. We lost. Based on current knowledge, I will vote against reconsideration. I don't consider that a vote for him at all," Evans told United Press International in explaining his position.

Evans was one of five Republicans who opposed Manion in last month's 48-to-46 vote to approve the nomination, marked by charges of misrepresentation and vote-trading.

A switch by Evans would mean that Washington state's senators had provided the margin for confirmation. Sen. Slade Gorton (R-Wash.) switched his vote in Manion's favor last month after the White House promised to approve a district court nominee in his state. There has been speculation that Evans may be trying to defuse the controversy involving Gorton, who faces reelection.

A defection by Evans on the procedural motion -- which is to table a Democratic move to reconsider last month's vote -- would leave opponents with 49 or 50 votes, according to Senate sources. A 50-to-50 tie, which would result if opponents pick up the vote of undecided Sen. Bob Packwood (R-Ore.), would be broken in Manion's favor by Vice President Bush and make his confirmation final.

Dole announced yesterday that he would call up the nomination today but added, "If someone disappears, that could change." He has said he will allow a vote only if he is sure that Manion will be approved.

Critics charge that Manion, a conservative Indiana lawyer, is too inexperienced and ideologically extreme for the 7th Circuit Court of Appeals in Chicago, while supporters say opposition is based on liberal ideology. The battle has escalated into a debate on the quality of President Reagan's judicial appointments.

"We come out of this, whether we win or lose, clearly the winner," said Sen. Joseph R. Biden Jr. (D-Del.), a Manion opponent. If Manion is approved, "Nobody has any doubt about why he's getting confirmed. They have no doubt it's a political sellout."