ON HIS WAY across the country to Santa Barbara, President Reagan made a campaign stop in St. Louis for Senate candidate Christopher Bond. Mr. Reagan spoke from a hotel built with a UDAG grant -- one of the programs he would zero out. He advocated free trade -- a cause not exactly championed by Missouri's Republican Sen. John Danforth who was traveling elsewhere. He denounced "big taxers and big spenders" -- while calling for a continued majority for the same Senate Republicans who have pushed for taxes to close the deficit.

For connoisseurs of political irony, plenty to feast on here. For those seeking instruction in how control of the Senate will be determined, not so much. Republicans hold 22 of the 34 Senate seats up this year and must win 18 races to maintain control; most experts give them a 50-50 chance to do so. But their calculations hinge not so much on an assessment of national trends as a state-by-state analysis of local factors.

No one thinks the president's popularity is transferable, and many Republicans fear that in critical states the unpopularity of his trade, farm and tax proposals may be. They like the concept of Gramm-Rudman (originally a Senate Republican initiative), but they are not eager to be seen next October supporting cuts in Medicare, Medicaid and student loans.

So they are following the lead of Sen. Charles Grassley (R-Iowa) and adapting their campaigns to the local terrain. That means the outcome of the battle for control of the Senate will depend on the outcomes of each of the 34 Senate races. The odds seesaw as oddsmakers watch the local polls oscillate and decide whether to run. Since December, Democrats seem to have gained some ground in five states (Vermont, Missouri, Wisconsin, California, Georgia) and Republicans in five (Oklahoma, North Dakota, New York, Pennsylvania, Louisiana). Purely personal factors can matter. Robert Kasten loses ground in Wisconsin after a drunk driving arrest; analysts wait for the reaction to the announcment that Florida's Paula Hawkins might need spinal surgery.

The irony is that this year's crop of senators was chosen in 1980 when, more than in any recent year, the contest came down to a choice between two parties and their philosophies. In contrast, the nation's choice in 1986 seems likely to come down to 34 choices between two individuals, a choice less between national philosophies than between personal talents and idiosyncracies.