In the working-class neighborhoods of Baltimore, where sample ballots distributed by Democtratic political clubs sway thousands of votes, rank-and file Democrats wll be urged next Tuesday to vote the straight ticket: Jimmy Carter for President, Parren J. Mitchell (or Barbara A. Mikulski or Clarence D. Long) for the House of Representatives -- and Republican Charles McC. Mathias for the U.S. Senate.

Mathias, the highest-ranking Republican in Maryland, is on those sample ballots party because many Democrats cannot support the conservative candidate of their own party -- Edward T. Conroy -- and partly because Mathias has made a deliberate effort to serve as a financial underwriter for the Democratic tickets in the state's largest city. He has donated nearly $10,000 in campaign funds to Baltimore's Democratic clubs to help them print thousands of election day sample ballots with his name nested snugly between Carter and other Democrats.

The liberal Republican senator, who is seeking a third term, has courted traditinal Democrats in previous elections, but never before has he done it so brazenly, and never before have the Democratic organizations in Baltimore so openly embraced him and his money. Already, a half-dozen Democratic clubs have accepted Mathias' offer to pay for part of the cost of printing more than 75,000 sample ballots.

If this unusual bipartisan strategy helps President Carter, so be it, say aides to Mathias. Until recent weeks, Mathias would not even publicly endorse the conservative presidential nominee of his own party. These days. Mathias will -- if pushed -- month the phrase "I support the Reagan-Bush ticket," but there is little enthusiasm in his words.

While Conroy and his aides chafe at the arrangement, the Carter organization, which offered to pay the full cost of ballot printing for clubs that endorsed the straight Democratic ticket, seems comfortable with the split-ticket deal. In instances where clubs are support Mathias, the Carter campaign still offers to pick up one-third of the tab.

Allen Levey, the state Republican Party chairman, said he "disapproved" of the Mathias arrangement, but added: "I know it goes on."

Mathias strategists concede there is ome irony in the Republican senator boosting of fortunes of an otherwise straight Democratic ticket, including one candidate, Mikulski, who unsuccessfully attempted to unseat him in his last campaign. But they insist that the reason for joining hands with the big-city Democratic machine is to enhance Mathias' reelection chances.

Three different clubs in Baltimore's 3rd Congressional District, where Mathias lagged behind Mikulski in his 1974 race, are distrubting ballots and letters bearing the names of both Mathias and Mikulski, who now represents the district in Congress. Both candidates are contributing to the cost of the ballots, and officials of two of the clubs said that Mathias' campaign will help pay for the free lunches they will distribute to their poll workers on election day.

Despite the defections, most of the powerful Democratic organizations on the south and east sides of the city -- where sample ballots carry the most weight -- are staying with their party's nominee.

For those clubs, party loyalty is still an inviolate code. State Sen. Harry (Soft Shoes) McGuirk, a premier organization leader in South Baltimore, noted gruffly, when told that Rep. Mitchell was backing Mathias, "Well, he [Mitchell] says he's not running again [after this year]."

The 7th District member of Congress, a member of black Baltimore's Mitchell dynasty, has recorded commercials for Mathias that are being played on four Baltimore radio stations in the closing days of the campaign.

Mikulski is ostensibly supporting Conroy. Her aide, Ann Lewis, said she "took Conroy to a couple of meetings" in her 3rd Congressional District. But Mikulski has made no attempt to dissociate herself from the Carter-Mathias-Mikulski sample ballots because "Barbara doesn't believe in dictating to people how to vote, "Lewis said.

Another club, the Fifth District Reform, whose members include House Speaker Benjamin Cardin, Attorney General Stephen Sachs and the Democratic state chairman, State Sen. Rosalie Abrams, did not print a ballot this year for fear of embarrassing Abrams: If the club had ordered a ballot, it would have had Mathias' name on it.

"I have no knowledge of that," said Abrams, who added, "there may be a lot of ticket-splitting in this election and I just hope all the Democrats come out and vote for Carter."

Another club, the Third District Citizens, overrode the recommendation of its own board of directors to endorse Mathias after Mathias and Conroy debate before the group.

The Metro Democratic Club in west Baltimore also listened to Conroy and then chose Mathias, according to treasurer Gwendolyn Cook, because "Mathias has done a very creditable job and Conroy is really very conservative on the issues of abortion, nuclear energy and defense."

"Conroy came before us," recalled state Del. Steve Sklar of the liberal New Democratic Coalition Club in northwest Baltimore, "and asked us whether we wanted to re-elect a senator who had opposed the B-1 bomber, who had voted for abortions, and most of them nodded their heads and voted accordingly."

Now, Mathias is contributing one-third of the club's costs for ballot printing and election day work, while the Carter campaign is putting up another third.

In a classic role-reversal, Richard Scott, press spokesman for the Democratic nominee, insinuated that the Republican incumbent was perpetuating a thinly veiled 1980 version of the outlawed "walk-around money" scheme.

Mathias' press secretary, Jack Eddinger, responded, "We have no intention of distributing walk-around money, although we did that in the 1974 campaign, when it was still legal."

Eddinger said the Mathias campaign is prepared to spend $10,000 to $12,000 for a late get-out-the-vote drive in key Baltimore precincts, but that none of the money would go to pay for workers, as has been the practice in the past.

Conroy wants Mathias to go further. His aide, Scott, said, "We have issued a statement that we will not offer any money to any political club in Baltimore or anywhere else in the state of Maryland and I challenge my opponent to do the same thing."

Conroy's good-government claim may be partly related to economic reality. State Sen. Joe Bonvegna, an East Side Democratic boss who is staying with Controy, observed of Conroy, "He ain't a got a dime."