United Auto Workers members with General Motors picket outside of one of the automaker's truck plants in Flint, Mich. on Sept. 16. The union called a strike against the automaker after a deal was not reached by a Saturday night deadline. Michael Wayland / CNBC

DETROIT – Pressure is building on General Motors and the United Auto Workers to reach a deal to end the union's strike against the automaker, now in its 19th day. Shares of GM have plunged by double digits since the strike began amid a weakening auto sector; vehicle production has ceased at nearly all North American plants for the company, including its highly profitable SUVs and pickups; and the work stoppage could soon begin having a more substantial impact on dealers. "They've got more pressure on both sides, but GM is beginning to face a lot of financials," said Art Wheaton, a labor expert at the Worker Institute at Cornell University. He added, the current public sentiment "is overwhelmingly on the UAW side," potentially putting additional pressure on the automaker. GM has enough cash and inventory to wait out the strike for months, however it's losing out on time to make up for lost production – specifically on pickups and SUVs – each hour the strike continues, a headwind for the automaker's third- and fourth-quarter earnings.

Running the math

Morgan Stanley analyst Adam Jonas said "investors are running the math on the tradeoffs between the up-front pain of lost profit and cash flow vs. potential concessions to the union that could impact GM competitiveness longer term," according to a research note Wednesday. Investors seem to be losing their patience. GM's stock has fallen by about 10% since the start of the strike. It closed at $34.98 Thursday, down from $38.86 on Sept. 13 — the Friday before UAW leaders called for the strike. It's also hitting shares of Fiat Chrysler and Ford Motor since GM's labor contract will serve as an outline for negotiations at its crosstown competitors. Fiat Chrysler's stock also tumbled 10% during that time period, while Ford Motor has declined 8%. The broader Dow Jones Industrial Average fell by 3.7% and the Nasdaq OMX Global Auto Index fell by about 6.6% over the same time frame. J.P. Morgan estimated the UAW's strike cost GM more than $1 billion during the third quarter, according to a note to investors Monday. Barclays analyst Brian Johnson last week estimated there could be 100,000 units of lost production in the third quarter, costing roughly $750 million for GM. GM's ability to ship parts to dealers also has been severely hampered. The lack of parts is a growing concern for dealers since servicing and repairing vehicles is typically the most profitable part of a dealership.

Union impact

For the UAW, the longer the strike lasts, the more members' expectations for the contract could grow. That could lead to a rejection of a potential deal if union leaders can't deliver a better offer than the one that included more than $7 billion in investments and thousands of new jobs from GM over the next four years. "Absent any movement by the UAW on any cost offset proposals that have been tabled by GM, I am not convinced that the UAW strikers will see any improvement in the economics that have already been proposed," Colin Lightbody, a labor consultant and longtime negotiator for Fiat Chrysler, wrote in a blog this week. Every day the strike goes on, according to Lightbody, there is a "really good chance that the UAW striker's return on investment is diminishing." The union this week also started paying its roughly 48,000 striking GM members $250 a week in "strike pay" out of the UAW's "Strike and Defense Fund," which totaled nearly $800 million before the strike. While well funded, the union is paying up to roughly $12 million a week to the striking members. To receive the payments, members must be in good standing and actively picketing to receive the pay.

United Auto Workers members on strike picket outside General Motors' Detroit-Hamtramck Assembly plant on Sept. 25, 2019 in Detroit. Michael Wayland / CNBC

UAW spokesman Brian Rothenberg, in response to questions about pressure mounting for a deal, said the union's "members stood up for GM when they needed us and now they need GM to stand up for them when they need them." GM declined to comment directly on the talks, referring to a previous statement that it remains "committed to reaching an agreement that builds a stronger future for our employees and our company." Despite many members saving in preparation for a strike, $250 a week is a financial burden and far less than the weekly compensation members usually receive. The starting wage for hourly assembly workers ranges between roughly $16 to $30 an hour, or $640 to $1,200 a week, before taxes and any overtime pay.

Which side blinks first?