Why a shutdown might happen
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The deadline for funding (the end of the fiscal year) is September 30. If Congress doesn’t pass appropriation bills or a “continuing resolution” (a temporary funding bill), parts of the government will lose funding. Brookings+3CRFB+3News 9+3
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There is disagreement between Republicans and Democrats over what the temporary funding bill should include. For instance, Republicans favor a “clean” continuing resolution (i.e. minimal changes from current funding levels), while Democrats are pushing for restoration or extension of healthcare subsidies, Medicaid funding, and some policy riders. Wikipedia+4Wall Street Journal+4Wikipedia+4
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Some stopgap funding proposals have already failed or stalled in the Senate. That increases the chance that nothing gets approved in time. Reuters+2Wikipedia+2
Why a shutdown might be avoided
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Often, political pressure and public backlash push lawmakers to reach at least a temporary agreement.
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There are legal and practical reasons (e.g. federal employees, critical services) that motivate both sides to find a workaround.
My estimate
Given the current state of negotiations and the failed proposals so far, I think it’s more likely than not that a partial government shutdown happens around October 1, unless a compromise gets made very soon.
