CHICAGO -- It's now or never for the St. Louis Cardinals.

This week's visit to Wrigley Field for a four-game series against the Chicago Cubs may represent their last chance to start closing an imposing gap with the National League Central leaders.

Or it may signal a realization that the more practical objective is an NL Wild Card berths.

The series opens Thursday as the Cardinals send right-hander Carlos Martinez (10-7, 3.29 ERA) against left-hander Jon Lester (12-4, 2.93 ERA).

Martinez is coming off a 13-5 loss Saturday night to Atlanta, giving up seven runs over five innings, including a pair of three-run homers. Lester, who has three wins and two no-decisions in his last five outings, faces St. Louis for the first time this season .

Just before the All-Star break the Cubs were mired in a 6-15 slump while the Cardinals had closed to within seven games of the division leaders. And with 10 head-to-head games ahead in the second half, St. Louis was thinking about a potential run.

"The Cubs have a great team and it would still be a big challenge to catch them if their lead was four games instead of seven," Cardinals second baseman Matt Carpenter told reporters in mid-July. "It's not going to be easy -- but it does at least seem more realistic than it did a few weeks ago."

That was then.

Three-plus weeks later Chicago owns a 12-game lead on second-place St. Louis. The Cubs have gone 19-6 since a victory just before the All-Star break and now have won nine straight.

The Cardinals (60-54) are 4-6 in their last 10 games after Wednesday's 3-2 victory over the Cincinnati Reds.

The Cardinals come into the series four games behind Los Angeles for the top wild card spot and tied with Miami for the second.

"Every game for us now is important.," said Jhonny Peralta on Wednesday. "Today was a big win. Miami lost and we won, so we are good to go. Now we go out there and try to win another game."

The Cubs became the first big league team to 70 wins with Tuesday's 5-1 victory over the Angels. They maintain baseball's best record and are on track for 102 wins.

Chicago is -- as pitcher Jake Arrieta put it last week -- "operating on all cylinders." Among the differences over the last 24 games entering Wednesday was better performances and consistency from Arrieta and other pitchers.

Cubs pitchers had a collective 2.37 ERA in that span, including a 2.61 ERA from starters and a 1.76 ERA from relievers. In 24 games prior, the Cubs had a 5.17 ERA

Not even a soap opera involving bench player Tommy La Stella and his refusal to report to Triple-A Iowa has distracted the Cubs.

The left-handed hitting infielder was sent down to Iowa 12 days ago but was reported to be instead working out at his New Jersey home and Cubs brass put La Stella on the inactive list on Tuesday.

"He's a 27-year-old kid and he's working through some stuff and we're trying to give him time to work through those things," Cubs general manager Jed Hoyer said this week. "We've had a lot of contact with him and we'll continue to do that."