Washington — For most blacks today there is only one party, the party of Franklin Roosevelt and John and Robert Kennedy: The Democratic Party. The Hispanic voters are, for the most part, following a similar voting pattern.
Yet Republicans are always talking about broadening the base of their party. President Reagan now calls this ''reaching out'' a prime political objective - after hearing that he is facing an almost solid, and sometimes passionate opposition from those in black and Hispanic communities.
Sen. Charles H. Percy of Illinois is one Republican who, from the beginning of his years in the Senate back in the mid-'60s, has had no difficulty at all in attracting these and, in fact, nearly all minorities to his side.
Senator Percy also has the women's vote, which is now against Mr. Reagan. Percy is well liked among East European ethnic groups. And, despite incurring unhappiness among some Jewish leaders over what many observers have called his evenhanded approach to the Mideast, Percy still enjoys far more support among Jewish American voters than do most other GOP candidates.
Recently Percy's support among blacks has been receiving quite a test. The senator has been in Chicago campaigning for Bernie Epton, the white, Republican mayoral candidate who is running against black Democrat US Rep. Harold Washington.
Yet Percy, in talking to black leaders on Chicago's South Side, says he has found them understanding of his support of Epton. They are telling him that it is only natural and right for a Republican to be backing a Republican - and that they are still with him.
Further, a new Robert Teeter poll of Chicago voters shows that 53 percent of the blacks are still in Percy's camp.
''I was astounded,'' Percy told reporters over breakfast. He said he had thought his opposition to Mr. Washington might have caused his favorable rating among blacks to plummet.
It has been Percy's strong and consistent civil-rights record, dating back to the 1950s, that has won him black - and now Hispanic - support. Further, through the years Percy has walked the black ghettos, shaking hands, showing he cares. Few Republicans in any big city in the country make this effort to show interest in black problems.
At one point in the breakfast, he was asked: ''Why aren't you a Democrat? He laughed and said, ''I wouldn't want to give some of the extreme right wing the satisfaction.''
Then he added: ''I think I can do more inside the Republican Party to keep it in the center of the road. That's where (President) Eisenhower was. And I'm an unabashed Eisenhower Republican. Eisenhower said, 'I like the center of the road: I don't want to be in the gutter on either side.' ''
Percy said that President Reagan ''has many political strengths, of course, that I don't have.'' He said he thinks Mr. Reagan's standing with minorities is ''recoverable.'' And he said, ''I think the President is going to work mightily to recover support among the minorities.''
The President recently spoke in behalf of Percy at a fund-raiser for the senator in Illinois, a presidential appearance that evoked considerable wrath among many of the GOP conservatives who oppose Percy.
''I did record what the President has to say about me at that speech,'' said Percy. ''And I may end up using it sometime.''
''It ain't going to hurt you, will it?'' he was ribbed.
''Not in a primary,'' he laughed.
Here Percy seemed to be saying that the President's support in a general election might have a mixed result - helping him among Republicans but, perhaps, hurting him among Democrats and independents.
''The Republican Party,'' Percy said, ''just isn't held in high repute in the black community. Under Bill Brock (Republican National Committee chairman during the Carter years) ''we were reaching out to broaden the base of the party. We have to go back to that.''