Once again the issue of tightening the nation's borders is being argued on Capitol Hill. Declaring that 500,000 illegal aliens will this year join some 3.5 million to 6 million already estimated to be here, Attorney General William French Smith has urged Congress to act fast to pass an immigration reform bill that failed last year. Sponsors believe that if it isn't passed quickly it is likely to be put off a couple of years.
The bill passed the Senate last year, 80 to 19, but failed to get action in the House. It is sponsored by Sen. Alan K. Simpson (R) of Wyoming and Rep. Romano L. Mazzoli (D) of Kentucky and is perhaps the most important immigration law since 1965. America, founded by immigrants, has what some call the most generous immigration law in the world.
But with a global recession, over 11 million US workers unemployed, an undermanned border patrol, and a tide of would-be immigrants, some see an emergency. Mr. Smith told a Senate committee that current restrictions ''have proven inadequate to meet the pressure of ever-increasing illegal immigration that even now threatens to engulf us.''
The Senate bill has several revolutionary changes for immigration laws.
* For the first time it imposes sanctions on employers who knowingly hire illegal aliens.
* It would grant amnesty to several million illegal aliens already here. They would have to wait five years before becoming citizens and would at first be ineligible for federal welfare payments.
* It would limit to 450,000 the number of immigrants who are not political refugess entering the country annually.
* It would facilitate admission of temporary foreign (guest) workers.
* It would increase the border patrol.
The highly emotional measure touches many rival groups and is testing the ability of Congress to legislate. Last year it ended up on the House floor with some 300 pending amendments. The Senate seems likely to pass the measure this year, but chances in the House remain uncertain. Unless action is quickly taken, it may become involved in the presidential race.
Here are elements involved in the argument:
* The American Bar Association convention last month strongly opposed so-called ''employer sanctions.''
* The NAACP endorses the sanctions, as does organized labor. US workers are losing jobs to aliens, they assert.
* Hispanic groups and the US Chamber of Commerce oppose sanctions against businesses; responsibility to control immigration belongs to the government, they say.
* Civil liberty groups fear federal registration. (Mr. Smith said that he would not encourage a national identity card system).
Meanwhile, both sides studied Mr. Smith's words Monday.
''We have lost control of our own border,'' he said. ''Failure to act can only result in further illegal migration, greater public frustration . . . and the negative effects . . . of so large a number of persons living outside the law.''
In effect, he said it's now or never.