Sometimes, covering politics is no more complicated than listening to what the people in politics say. Sometimes, they know what they are talking about.

Back on Jan. 6, a couple of us paid a call on Vince Breglio and Susan Bryant, who run the Republican Senatorial Campaign Committee. My notes from that day include this sentence: "Senate races will provide the real referendum on Reaganomics."

Five months later, with about half the Senate nominations settled, it is clear that Breglio and Bryant were right.

But don't take my word for it. Ask Leon Billings, the director of the Democratic Senatorial Campaign Committee. "The public is going to have a clear choice this fall," Billings said the day after last week's final big batch of spring primaries. "Almost all the Republicans are pure or relatively pure on supply-side Reaganomics. And almost all of our candidates will make that the issue of their campaigns."

They are already doing it. The night he was nominated, Frank Lautenberg, the businessman who captured the Democratic nomination in New Jersey and will face Rep. Millicent Fenwick (R) in November, put it this way:

"The voters have a clear choice between a staunch supporter of Reaganomics and a staunch supporter of what's good for New Jersey."

In California, Gov. Jerry Brown (D) began his uphill fight for the Senate against San Diego Mayor Pete Wilson (R) with a full-scale assault on the "disastrous, unfair economic policy" of Reagan. So obvious was Brown that Wilson told him in their first joint appearance, "Jerry, no matter how you try to run against Reagan, it's me against you."

It's not surprising to find sharp partisanship on economic issues in urbanized states with big and diverse electorates. You didn't have to be a genius to figure that in states like Michigan, Ohio, Pennsylvania and Tennessee, which went Democratic in 1976 and Republican in 1980, and where unemployment is high, Reagan's economic policies were bound to be at the center of the debate.

But who was to know that in Montana, Sen. John Melcher (D) would draw a Republican challenger, Larry Williams, who is an investment counselor and the author of a book titled, "How to Prosper in the Coming Good Years"?

And who was to know that in Virginia, where moneyed gentlemen usually arrange for both parties to nominate safe conservatives, the consensus would end with the retirement of the Senate's lone Independent, Harry F. Byrd?

Rep. Paul S. Trible, Jr., the Virginia GOP nominee, pledged to uphold "the time-honored conservative principles." But Lt. Gov. Richard J. Davis, the Democratic senatorial choice, said more than "me, too." He said, "It is unsatisfactory to me that we have the highest unemployment rates since the Depression, the highest interest rates internationally since the time of Christ and the highest rate of business bankruptcies."

So, suddenly, Virginia was added to the list of states where the Senate candidates were offering a choice--and not an echo.

In this instance and others, it is the Democrats who are pushing the economic issue to the forefront. But Reagan and the Republicans also deserve credit for making 1982 a significant referendum year.

Reagan set his economic program as the centerpiece of his 1980 campaign and for 17 months has kept it at the top of the political and governmental agenda. He has used his persuasive powers and the Republican Party's resources of money and organization to keep Republican legislators lined up, for the most part, behind Reaganomics.

Even those Senate hopefuls who might be tempted to stray have found it impolitic to do so. Despite the current economic strains, Reagan retains a hard core of support in the country. Among those who vote in Republican primaries, loyalty to Reagan is still a litmus test of acceptability.

Wilson found that to be the case in California, and Fenwick in New Jersey. Both of them are moderate Republicans who backed Jerry Ford over Reagan in 1976. But in order to defeat primary opponents with better Reagan credentials, both Wilson and Fenwick had to identify themselves strongly with Reaganomics. And their Democratic opponents will not let them forget.

The reverse side of the coin can be seen with some of the Democratic senators running in 1982. Last year, such men as Lloyd Bentsen of Texas, Robert C. Byrd of West Virginia and John C. Stennis of Mississippi voted with Reagan on some of the key economic roll-calls.

But all of them have been outflanked on the right by Republican challengers so much more ardent in their advocacy of Reaganomics that, willy-nilly, the Democrats look like critics of the President.

The result is, as Billings said, that "at least on the economic issues, we're going to have party politics" in this autumn's Senate elections. And that debate can serve the country well.