They've trended to historical lows in the last couple of years, but in the 1980s they'd get up to a fifth of the vote.
Their highest vote share (as IU) was in the 1994 European elections, where they polled 13.44%. In a General Election their highest result was 10.54% in 1996, which was also theit highst-ever vote count at 2.6 million.
The highest result of the Communist Party was in the 1979 municipal elections with 12.7%, and their highest General election result was 10.77% in 1979. In think IU was created in response to the PCE's electoral collapse in 1982 (4% of the vote - but that's the year of Felipe González's landslide win with the PSOE). By laying out pros and cons we risk inducing people to join the debate, and losing control of a process that only we fully understand. - Alan Greenspan
Prior to the breakup between the unions and parties in the late 1980s, the strength of the PCE was bolstered by the number of works council meetings won by the CCOO. They had a social base. The same was true for PSOE, but at the start in the union elections CCOO was stronger than UGT (connected to PSOE).
I'm more interested in that than electoral politics. And I'll give my consent to any government that does not deny a man a living wage-Billy Bragg