Joseph C. McGrath, the Republican candidate for Montgomery County executive, would love to spend the last four weeks of the campaign talking trash, but the Democratic incumbent, Charles W. Gilchrist, keeps bringing up President Reagan.

Just about the time banker McGrath wants to discuss the high cost of disposing of refuse, or the long delays and costly studies that preceded the opening of the controversial new county landfill, Gilchrist tells audiences, such as the League of Women Voters last week, how the national Republican administration is "turning its back" on the poor.

Gilchrist also likes to talk about all the things his administration is doing to respond to federal cutbacks, such as finding state money to replace federal funds for Metro and enlisting private lawyers to replace the legal services program.

McGrath, like some other Republican candidates locally and statewide, has been dogged in this campaign by "the Reagan issue." Yesterday, he held a press conference at the county's GOP headquarters where, before photographs of past Republican presidents, he tried to refocus the campaign back to local concerns.

"I have come before you today," McGrath proclaimed, "to pierce the Reagan smokescreen behind which our county executive has hidden since he won the Democratic primary." McGrath said "the real issue" of the campaign is Gilchrist's leadership. He attacked the incumbent for being indecisive, unable to make timely decisions and for politicizing the county's professional civil service system.

Gilchrist denied hiding behind a "Reagan smokescreen." "I have not used that as a campaign strategy," he said, although he said "it's legitimate" to point out where Reagan's cuts have hurt the county and how he has responded with specific programs.

Many Montgomery Democrats, like Democrats around the country, think the president and his economic policies are among their chief assets going into November. Shortly before McGrath's press conference, Rep. Michael D. Barnes of Montgomery held his own press conference to announce the results of his latest poll that he said showed "it's going to be a tremendous year for Democrats."

The poll of 411 probable voters in Barnes' 8th Congresssional District was conducted by Potomac Survey Research, which is owned by former unsuccessful Democratic congressional nominee Lanny Davis. It found that 51 percent of those responding gave Reagan a negative job performance rating while 44 percent rated the president positively.

In his race, the poll showed Barnes beating Republican Elizabeth Spencer 64 percent to 19 percent, with Barnes holding on to 81 percent of the Democrats, while picking up 59 percent of the independents and a third of the Republican voters.

Barnes attributed at least part of his solid Democratic backing to the Reagan factor. "It will be very hard for a Republican to win in this atmosphere in Montgomery County," Barnes said.

Spencer's campaign manager, Quinn Scamahorn, said "We've said all along it's going to be a tough, uphill battle." He added, however, that Reagan's economic policies should not be a factor "especially since Elizabeth doesn't subscribe to Reagan's economic dogma."

But unlike Spencer, McGrath proudly subscribes to most aspects of Reagan's economic program.