THE EQUAL RIGHTS AMENDMENT is still three states short with just nine months to go. Right now the crucial battleground is Illinois. Last week a ratification vote in that state's House of Representatives fell slightly short, largely because it got entangled in a leadership dispute among black delegates from Chicago. This week the pro-ERA forces are going to try again. If they can finally muster a three-fifths majority, required by an extraordinary state rule, the fight will shift to the state Senate. A major factor there will be Republican Gov. James Thompson's ability to line up enough GOP votes to carry the ERA through.

If the strong, bipartisan pro-ERA campaign in Illinois should fail, the chances for victory in three other states before March 22, 1979, would be bleak. Even with Illinois, the prospects elsewhere would be uncertain at best. Thus, while redoubling their efforts in the field, many of the amendment's backers are also asking Congress to extend the ratification deadline to 1986. A House Judiciary subcommittee endorsed that by a 4-to-3 vote last week.

Perhaps in seven more years the idea of ending discrimination on grounds of sex will become irresistible in North Carolina, Florida, Missouri, Virginia and other states where the ERA has not yet prevailed. But extending the deadline is, in our view, a bad idea. It smacks of the same kine of expedient rules-changing that anti-ERA forces have managed in some states - and pro-amendment groups rightly resent.

Congress has put time limits on recent proposed amendments for good reason: to ensure that ratification, if achieved, will reflect a national consensus on a fundamental constitutional change. If the time period for approving ERA is going to be extended at the last minute, surely states that have already ratified should be allowed to reconsider and, if they wish, rescind those votes. The pending legislation would not permit that.

We think advocates of the ERA should regard next March's deadline - however frustrating - as firm, and concentrate on their campaigns in the states. They still have time - and, in most of the balky states, this fall's elections - to show that their cause does have substantial public backing and political force. Indeed, the deadline ought to help them persuade some key state legislators that there's no time left for shyness, procrastination or political sporting with the ERA.